Power plant installation



Nov. 3, 1964 w. H. CLAYTON ETAL 3,155,076

POWER PLANT INSTALLATION Filed Dec. 27, 1961 3 Sheets-Sheet 1 63 :l-IJ

I 36 L 70 I 54 50 a I I u III" I I I I 22 I I 44 66 I 52 I I 56 4 I 402e 5:3 I

I I a 42 I I I g H I Q 52 54 i I I .24 g I g I I II B I I I Q I I I l II I g 72 l I I I R 54- I 12 I 54 l 7 I I lsz I i I; II MEI I I 52 l 8674 5 TD I l i k Y 78 FIG. I I I 92 I J INVENTORS:

ATTORNEY Nov. 3, 1964 Filed Dec. 27, 1961 7., CHANGE IN HEAT ABSORPTION-DECREASE Q INCREASE CHANGE IN HEAT ABSOPTION INCREASE- DECREASE o w. H.CLAYTON ETAL 3,155,076

POWER PLANT INSTALLATION 3 Sheets-Sheet 2 LOW PRESSURE REHEAT HIGHPRESSURE REHEAT ECONOMIZER FURNACE BURNER TILT INCREASE FIG. 2

ECONOMIZER HIGH PRESSU REHEAT LOW PRESSURE REHEAT FURNACE o GASRECIRCULATION FIG.3

INVENTORS WILLIAM H. CLAYTON WILLBURT W. SCHROEDTER ATTORNEY Nov; 3,1964 Filed Dec. 2'7, 1961 w. H. CLAYTON ETAL 76 POWER PLANT INSTALLATION3 Sheets-Sheet 5 Z 9 E 0: HIGH PRESSURE 8 \Z REHEAT I: "I" .1 owPRESSURE ,S REHEAT E2 0 v. LOAD v FIG. 4

2 Q E 0: HIGH PRESSURE 8 REHEAT G) E 5: 5 Low PRESSURE REHEAT 0 LOADFIG. 5

INVENTORs: WILLIAM H. CLAYTON B\{ VILLBURT W. SCHROEDTER ATTORNEY UnitedStates Patent" Oflice Patented Nov. 3, 1964 3,1553% PQWER PLANTKNSTALLATHON William H. Slay ton, Windsor, and Wilihurt W. Schroedter,

West Hartford, Conn, assignors to (Iomhustion Engineering, Ina, Windsor,Conn, a corporation of Delaware Filed Dec. 27, 195i, Ser. No. 162,330 8tCiaims. (Q1. 122-479) This invention relates generally to a power plantthat is comprised of a vapor generator-turbine unit or combination andhas particular relation to such a power plant unit operating on thedouble reheat cycle.

In accordance with the invention there is provided an improved method ofoperating and an improved organization of such a double reheat powerplant system wherein the high pressure and the low pressure reheattemperatures are controlled by a combination of control effects oroperations organized in a manner and with relation to the systemarrangement to realize optimum operating results from each of thesecontrols. In the double reheat cycle with which the invention isconcerned the heat requirements of the high pressure and the lowpressure reheats vary relative to each other with varying load. it isdesired" that the two reheat temperatures, i.e., the high pressure andthe low pressure reheat temperatures, as deli 'ered to the intermediateand low pressure turbine stages or sections, respectively, be maintainedat a desired and generally constant value throughout the operating loadrange of the unit. Because of the aforementioned characteristic of thetwo reheats it is necessary to provide specific controls to effect thisresult. With the present invention the control of the two reheattemperatures is effected primarily by different control actions and as aresult of which the reheat temperatures are maintained at their desiredvalue.

This is achived by arranging the high pressure and low pressure reheatsurface relative to each other in the vapor generator in a manner sothat gas recirculation control has its greatest and primary etfect onone of the reheats, preferably the high pressure reheat, while theadjusting of the zone of combustion in the furnace has its primary orgreatest effect on the other reheat, preferably the low pressure reheat.Through this arrangement and method of operating the two reheattemperatures are controlled throughout the load range over which theunit is operated or at least a major portion of this load range.

Accordingly it is an object of the present invention to provide animproved power plant system wherein the vapor generator-turbine unit isorganized to operate on the double reheat cycle and wherein the tworeheat tempera tures are controlled over a substantial load range.

A still further object of the invention is to provide such an improvedpower plant system wherein control of two reheats is effective through acombination of gas recirculation type of control and the controlproduced by adjustment of the zone of combustion. in the furnace.

Other and further objects of the invention will become apparent to thoseskilled in the art as the description proce eds.

With the aforementioned objects in view, the invention comprises anarrangement, construction and combination of the elements of theinventive organization in such a manner as to attain the results desiredas hereinafter more particularly set forth in the following retailed'description or" an illustrative embodiment, said embodiment beingshownby the accompanying drawings wherein:

FIG. 1 is a diagrammatic representation in the nature of a verticalsectional view through a high capacity vapor generator-turbine powerplant organized in accordance with the present invention;

FIG. 2 is a graphical representation illustrating the control effectobtained by adjusting the zone of combustion in the furnace;

H6. 3 is a similar graphical representation illustration the controleffect obtained with gas recirculation control;

FIG. 4 is a graphic illustration of the heat absorption characteristicsof the two reheats of the double reheat organization of the inventionunder one preferred operating cycle; and

FIG. 5 is a similar graphic representation of the heat absorptioncharacteristic of the two reheats under a modified cycle of operation.

Referring now to the drawings, wherein like reference characters areused throughout to designate like elements, the illustrative andpreferred embodiment of the invention includes the furnace 1i into whichfuel and air are introduced by the burners 12 with the fuel being burnedin the furnace and the combustion gases thus generated passing upthrough the furnace and through the lateral outlet at the upper end fromwhich extends the horizontally directed gas pass 14 that is, in turn,connected with the downwardly extending gas pass 16. This latter gaspass extends downward in parallel, spaced relation with the furnace andthe combustion gases egressing from the furnace and after traversing thehorizontal gas pass 14, are directed down through this vertical gas passand out the outlet r8 to a suitable air heater and stack or other conventional equipment.

The primary fluid of the vapor generator is first introduced into andpassed through the economizer 2d with this fluid being supplied to theeconomizer through a suitable feed pump means, not shown. From theeconomizer the primary fluid is conveyed to the chamber 22 through theconduit 24. From this chamber 22 the fluid. passes down through theconduit 25 into which is connectedthe pump '28 and from pump 28 theprimary fluid enters the headers 3d at the lower end of the furnace.Connected with these headers 3t) are tubes 32 which line the walls ofthe furnace, with the furnace preferably being generally rectangular intrans-verse section and with the tubes 32 extending upwardly along theinner surface of the furnace walls and connecting with suitable headers34 at their upper ends. From the header 2- 5 the primary fluid isconveyed to the chamber 22 as through conduit 36. From this chamber 22primary fluid is conveyed to the heat exchange section 38 throughconduit 44 and from this heat exchange section 33 the primary fluid isfinally conveyed through conduit 42 to the high temperature vaporheating section 44 of the primary circuit of the vapor generator. Aftertraversing this heat exchange section 4 the primary vapor is heated toits desired temperature and it is then conveyed to the high pressurestage or section 46 of the turbine machine designated generally 48 withthe vapor being conveyed through conduit 5d.

From the high pressure stage 46 of the turbine the vapor is conveyedthrough conduit 52 to the low temperature section S t-of the highpressure reheater. After being partially reheated in this section thehighv pressure reheat vapor is conveyed through conduit 56 to the hightemperature stage 58 of the high pressure reheater and from this hightemperature stage the reheat vapor is con veyed through conduit 6% tothe intermediate stage es of the turbine.

The vapor is reheated a second time by being conveyed through conduit asto the low pressure reheater 66. After being reheated to its desiredtemperature this low pressure reheat vapor egresses from this heatexchanger es and is conveyed to the in et of low pressure turbine stage68 through the conduit 79. The exhaust from this low pressure stage isconveyed to the usual condenser and thence to the condensate systemwhich is conventionally found in vapor generator-turbine units to whichthe invention pertains.

As previously mentioned the heat requirement of the high pressure andthe low pressure reheats vary as the load on the unit changes. With theinvention the general characteristic prevails that as the load decreasesfrom maximum the percentage of heat absorbed required by the highpressure reheat vapor necessary to maintain this reheat vapor at itsdesired, generally constant, temperature throughout the load rangeincreases relative to the heat requirement of the low pressure reheaterin order to maintain the temperature of this low pressure reheat vaporat its desired, generally constant, temperature throughout the loadrange. Accordingly, in order to insure that the temperature of the highand the low pressure reheat vapor will be maintained as desired it isnecessary to provide control means to effect this result.

While the aforementioned general characteristic with regard to therelative heat absorption requirements of the low pressure and the highpressure reheats prevail with the invention, in One preferred embodimentof the invention the particular heat requirement characteristics of thereheats is of the nature depicted in FIG. 4 wherein, of the total heatabsorption of the unit both the high pressure and the low pressurereheats have an increasing heat absorption characteristic, i.e., withrelation to the total, the percentage of heat absorption of these tworeheats increases as the load decreases from its maximum value, with theincrease of the high pressure reheat being substantially greater or atan increased rate relative to that of the low pressure reheat. This isshown in the FIG. 4 curves wherein the slope of the high pressure reheatcurve is much greater than that of the low pressure reheat curve.

A modified arrangement and process of the invention with a somewhatdifferent characteristic is provided with this modified operation beinggraphically illustrated in FIG. 5. In this modified arrangement the highpressure reheat has a rising heat absorption characteristic withdecreasing load while the low pressure reheat has a decreasing heatabsorption characteristic with decreasing load. This is evident from theFIG. 5 plot wherein load is plotted against the percent of heatabsorption with relation to the total and wherein the high pressurereheat curve has a rising slope as the load is decreased and the lowpressure reheat has a falling or downward slope.

The characteristics thus graphically illustrated in both FIGURES 4 and 5are a result of the particular turbine hookup and design as incorporatedinto the power plant system.

In accordance with the invention a combination of gas recirculationcontrol and control by means of varying the zone of combustion in thefurnace is provided and two reheats are arranged in a manner so that thetemperature of each may be regulated throughout a desired load rangethrough the action of these control systems.

Adjustment of the zone of combustion within the furnace is preferablyachieved by means of firing the furnace through tiltable burners whichmay be arranged symmetrically about the furnace walls at locations inthe burner zone and so as to direct fuel and air tangent to an imaginaryvertical, centrally located cylinder with respect to the furnace therebycreating a whirling mass of burning fuel in the furnace. Such a firingarrangement which will provide for adjustable variation of thecombustion zone with respect to the outlet of the furnace is disclosedin US. Patent No. 2,575,885 of November 20, 1951. In the illustrativearrangement of FIG. 1 the tiltable burners or burner nozzles aredesignated generally 72 and they may be pivoted so as to vary thecombustion zone. It will be understood that other means of adjustablyvarying the zone of combustion toward or away from the furnace outletmay be utilized such as providing burners in the furnace throughout thevertical extent of adjustment of the combustion zone and controlling thenumber and location of the burners that are in operation.

The effect of moving the zone of combustion from a region away from thefurnace outlet toward the outlet or, in the illustrative arrangementmoving the zone of combustion upwardly from zone A toward zone B, is toincrease the temperature of the gases in the upper portion of thefurnace and leaving the furnace with an opposite effect being realizedby the opposite adjustment of the zone of combustion.

The control effect that is obtained by means of varying the zone ofcombustion in the furnace is greatest near the furnace outlet andbecomes progressively less at locations further downstream relative togas flow from the furnace outlet. Accordingly, in the arrangementdisclosed the effect of varying the zone of combustion will be greateston the heat exchange sections 58 and 66 and will become progressivelyless on the heat exchange sections 38, 54 and 20. This is graphicallyillustrated in FIG. 2 wherein it is shown that the effect on the highpressure reheat is high while that on the low pressure reheat issomewhat lower and that on the economizer is still lower. Of course theeffect of elevating the zone of combustion in the furnace will cause adecrease in the percentage of heat absorption in the furnace.

The gas recirculation control which forms part of the invention includesthe gas recirculation fan '74 which is connected to the lower region ofgas pass 16 through duct f6 and has its outlet connected with the lowerregion or hopper of furnace It through ducts 78.

In contrast to the effect of adjusting the zone of combustion in thefurnace, the gas recirculation type of control has its maximum influenceor effect on heat exchange surface located most remote in a downstreamgas flow direction from the furnace outlet. Accordingly by introducingcombustion gases into the furnace the heat pick-up of the economizer isincreased to the greatest extent while the heat pick-up of each of theheat exchange surfaces progressively upstream of the economizer isincreased to a progressively lesser extent. This is illustrated in theFIG. 3 set of curves which show that the percentage of change of heatabsorption effected by gas recirculation is greatest on the economizerin the illustrative organization of FIG. 1, is somewhat less on the highpressure reheater and is still less on the low pressure reheater. Sincethe effect of the gas recirculation control as employed with theinvention is to decrease the heat absorption in the furnace, this heatabsorption will fall as depicted in FIG. 3.

FIGS. 2 and 3, respectively, show the over-all effect of gasrecirculation control and the control achieved by adjusting the zone ofcombustion in the furnace on the low pressure and the high pressurereheats. In order to obtain this characteristic it is necessary that thehigh pressure reheat be influenced predominately by the low temperatureportion 54 of the high pressure reheater so that, of the total heatabsorption of the high pressure reheat, a major portion is provided orobtained by this low temperature heat exchange section 54. Thus the highpressure reheat is elfectively downstream with relation to combustiongas flow of the low pressure reheat Wherefore the high pressure reheatmay be advantageously controlled by gas recirculation and the lowpressure reheat may be advantageously controlled by adjustment of thezone of combustion longitudinally of the furnace. Since gasrecirculation control is most effective at a location remote from thefurnace outlet while adjustment of the zone of combustion is mosteffective at a location close to the furnace outlet the control of thehigh pressure and low pressure reheat temperatures that is obtained bymeans of the present invention may be more sensitive by spacing the highpressure and the low pressure reheat from each other along thecombustion gas flow path and providing additional heat exchange surfaceintermediate the location of the high pressure and low pressurereheaters. Thus there is provided intermediate the low pressure reheater66 and the large low temperature portion of the high pressure reheater54 the primary heat exchange sur- $3 face 58. By arranging the heatexchange surface in the manner set forth the effective location of thelow pressure reheater'in the gas stream is close to the furnace outletwhile the effective location of the high pressure reheater is welldownstream of the low pressure reheater with primary heat exchangesurface being interposed between these two locations.

It will be appreciated that the high pressure reheater is divided into alow temperature section of high heat absorbing capacity and a hightemperature section of much smaller heat absorbing capacity for thereason that with the predominate portion of the heat absorption of thelow pressure reheat being provided at a downstream location thetemperature of the combustion gases at this location are sufilcientlylow so that it is necessary to provide a section of the high pressurereheater in a location of the gas stream having a high temperature inorder that the desired high final high pressure reheat temperature maybe realized. While the high temperature section 58 of the high pressurereheater is illustrated as disposed upstream relative to gas flow of thelow pressure reheater 66 this disposition of these heat exchangesurfaces may be reversed. In such a case a somewhat larger hightemperature section of the high pressure reheater will be required inorder to give the desired final temperature for the high pressure reheatvapor but an improvement in the control will be realized in that thehigh pressure reheater will become even more predominately responsive togas recirculation control and the low pressure reheater will become evenmore predominately responsive to the control obtained by adjustment ofthe zone of combustion in the furnace.

Since with the method and organization of the invention the heatabsorption of the low pressure reheat is changed to a much greaterextent than that of the high pressure reheat by adjustment of the zoneof combustion in the furnace while the heat absorption of the highpressure reheat is influenced or changed to a much greater extent thanthat of the low pressure reheat by the introduction of combustion gasesinto the furnace through the gas recirculation system the low pressurereheat temperature and the high pressure reheat temperature may beregulated and maintained at their desired value with varying loadthrough the combined adjustment of the zone of combustion and adjustmentof the recirculation of combustion gases. The control of these reheattemperatures through these control efiects may be automatically obtainedwith FIG. 1 diagrammatically illustrating an arrangement for achievingthis result and wherein there is provided temperature sensing device 8t?which responds to the temperature of the low pressure reheat vaporleaving the low pressure reheater 66 and being conveyed to the lowpressure stage 68 of the turbine machine 48. This device 89, through asuitable controller 82 which is interconnected with the control devices84 is effective to regulate the angular disposition of the nozzles '72and accordingly adjust the zone of combustion longitudinally in thefurnace toward and away from the furnace outlet. The temperature of thehigh pressure reheat vapor passing through the conduit 66 to theintermediate stage 62 of the reheater is sensed by the temperaturesensing device 86 which is effective to regulate the damper $8 throughthe action of controller 90 and the operating device 92. Through thesecontrols the temperature of the two reheats' may be regulated as theload varies to maintain the two reheats at a desired or predeterminedtemperature.

Since the high pressure reheat has a greater percentage change of theheat absorption than the low pressure reheat, as evidenced by FIGS. 4and 5, and since'the gas recirculation control provides a somewhatgreater control efiect than adjustment of the zone of combustion thepreferred arrangement is to have the high pressure reheat downstream ofthe low pressure reheat in the convection gas pass with regard to gasflow, with this arrangement being disclosed in FIG. 1. While this is thepreferred arrangement because a more effective utilization of the twocontrols or control operations may be had, the invention comprehends aninverse arrangement wherein the high pressure reheat is upstream of thelow pressure reheat in the convection gas pass and wherein thetemperature of the high pressure reheat is influenced predominately byadjustment of the zone of combustion and the temperature of the lowpressure reheat is influenced predominately by the recirculation ofcombustion gases to the furnace. In this inverse arrangement thetemperature of the high pressure reheat will be effectively controlledby adjustment of the zone of combustion in the furnace while thetemperature of the low pressure reheat will be effectively controlled byregulation of recirculation of combustion gases to the furnace.

In the FIG. 4 arrangement or modification the operation will be suchthat at maximum load on the vapor generator the zone of combustion willbe at its lowered range of adjustment in the furnace and there will belittle if any combustion gases recirculated through the combustion gassystem. It may be of advantage to pass just enough gases through thecombustion gas recirculation system in order to insure that there is notreverse flow through this system and in order to maintain the variouselements of the system at their operating temperature. With the unitthus operating at maximum load the high pressure and the low pressurereheat temperatures are at their desired value as for example 1100 F. Asthe load on the vapor generator-turbine unit decreases the high pressureand the low pressure reheat temperatures will tend to fall from theirdesired value. To offset this effect the zone of combustion in thefurnace is progressively moved toward the furnace outlet having apredominate effect on the low pressure reheat temperature in theillustrative or anization and the quantity of gas recirculated to thefurnace 1% is progressively increased influencing predominantly the highpressure reheat temperature with the two control actions being such asto maintain the two reheats at their desired value. Through thisoperation and system the two reheat temperatures may be maintained attheir predetermined value throughout a substantial load range.

in the PEG. 5 arrangement or modification the operation will be suchthat at maximum load on the vapor generator zone of combustion will beat its upper range of adjustment in the furnace or at an adjustment wellabove the lower range even if not at the extreme upper limit of therange of adjustment and as in the case of the PEG. 4 arrangement littleif any combustion gases will be recirculated to the furnace through thecombustion gas system. Here again it may be advantageous to pass justenough gas through the combustion gas system to insure that inverse flowthrough the system is not had and that the various elements of thesystem are maintained at their desired operating temperature. With theunit thus operating at maximum load the high pressure and the lowpressure reheat temperatures are at their desired value, as for example,1100" F. As the load on the vapor generatorturbine unit decreases theamount of combustion gases recirculated to the furnace is progressivelyincreased and the zone of combustion is progressively moved away fromits uppermost extreme position or the extreme position closest to thefurnace outlet toward the extreme position most remote from the furnaceoutlet. Through this combination of gas recirculation control andadjustment of the zone of combustion the two reheat temperatures aremaintained at their desired value throughout a substantial load range asthe load decreases from its maximum or percent rating.

Since the invention is concerned with regulating the two reheattemperatures in a vapor generator-turbine unit operating on the doublereheat cycle andv since the regulation of the primary vapor temperaturedoes not form a part of this invention this latter regulation has notbeen set forth although it will be appreciated that the primary vapordelivered to the high pressure turbine as through conduit 59 ismaintained at its desired value throughout the operating load range andthe regulation of this primary vapor temperature may be efiected throughany desired known means such as desuperheating or bypass dampers.

Accordingly it will be appreciated that with the present invention thehigh pressure and the low pressure reheat temperatures in a doublereheat unit may be advantageously and effectively regulated andmaintained at their desired value throughout a substantial load rangewith the regulation being obtained by means of regulated actionseffected in the furnace and with the organization of the generator beingsuch as to provide optimum temperature regulation of the two reheats.

While we have illustrated and described a preferred embodiment of ourinvention it is to be understood that such is merely illustrative andnot restrictive and that variations and modifications may be madetherein without departing from the spirit and scope of the invention. Wetherefore do not wish to be limited to the precise details set forth butdesire to avail ourselves of such changes as fall within the purview ofour invention.

What we claim is:

1. In a power plant installation a vapor generator operating in thedouble reheat cycle, said generator including an elongated furnacehaving heat exchange tubes on its walls and having a combustion gasoutlet, means firing said furnace in a manner such that the zone ofcombustion may be regulatingly moved toward and away from said outlet, ahigh pressure and a low pressure reheater means disposed so thatcombustion gases generated in said furnace pass thereover, means forregulatingly introducing com bustion gases which have traversed saidreheater means into said furnace at a location such as to increase theheating effect of the combustion gases passing over the reheaters overWhat it would be without said recirculation, said high pressure and lowpressure reheaters being positioned in the gas stream so that thecontrol effect obtained by gas recirculation is substantially greater onthe high pressure reheater than on the low pressure reheater while thecontrol effect obtained by adjusting the zone of combustion withrelation to the furnace outlet is substantially greater on the lowpressure reheater than on the high pressure reheater means operative tovary the recirculation of the combustion gases directly andpredominantly in accordance with the temperature of the high pressurereheat vapor and means operative to vary the zone of combustion directlyand predominantly in accordance with the temperature of the low pressurereheat vapor.

2. A vapor generator operating on the reheat cycle and wherein the vaporgenerator includes an elongated fluid cooled furnace having a combustiongas outlet adjacent one end, a gas pass extending from said outlet,means firing said furnace at a region remote from the outlet, a highpressure reheater and a low pressure reheater disposed to have thecombustion gases generated in the furnace pass thereover with the highpressure reheater being effectively downstream relative to combustiongas flow of the low pressure reheater, means for withdrawing combustiongases from the gas pass at a location downstream of the reheater andintroducing the same into the furnace at a location to increase the heatcontent of the gases egressing from the furnace, means responsivedirectly and predominantly to the temperature of the vapor leaving thehigh pressure reheater effective to adjustably regulate the combustiongases thus introduced into the furnace and means for adjusting the zoneof combustion toward and away from the furnace outlet and meansresponsive directly and predominantly to the temperature of the vaporleaving the low pressure reheater effective to regulate said means foradjusting the zone of combustion.

3. In combination a vapor generator operating on the reheat cycle andincluding an upright fluid cooled furnace having a combustion gas outletat its upper region, a gas pass extending from said outlet, meansintroducing fuel and air into the furnace at a region remote from saidoutlet, means introducing recirculated combustion gases into the lowerregion of the furnace, predominately connection heated high pressurereheat means and predomiately connection heated low pressurereheat meansin series flow relation with regard to the gas flow with the highpressure reheat means having a portion upstream of the low pressurereheat means and a portion downstream of the low pressure reheat meanswith the latter portion being of substantially greater heat absorbingcapacity than the former portion and disposed so that at least a majorportion of the combustion gases produced by the burning fuel in thefurnace pass thereover, heat exchange surface forming part of theprimary surface of the vapor generator and disposed intermediate the lowpressure reheating means and the downstream portion of the high pressurereheat means and means directly and predominantly responsive to the highpressure reheat temperature and operative to adjustably control thegases recirculated to the furnace and independently but simultaneouslytorn and directly and predominantly responsive to the low pressurereheat temperature and operative to independently but simultaneouslyadjustably vary the zone of combustion toward and away from the furnaceoutlet to provide independent control of the heat input of each of thereheats.

4. In a vapor generator operated on the reheat cycle the improved methodcomprising introducing and burning fuel in a fluid cooled furnacethereby creating a stream of combustion gases generating high pressure,high temperature vapor by imparting a portion of the heat evolved by theburning fuel to a working medium, passing said high pressure, hightemperature vapor to a working zone and utilizing a portion of theenergy thereof, reheating this vapor a first time by passing it in heatexchange relation with the combustion gas stream, conveying thisreheated vapor to a working zone and utilizing a further portion of theenergy thereof, thereafter again reheating said vapor by passing it inheat exchange relation with the combustion gas stream and conveying thisreheated vapor to a working zone, imparting at least a predominateportion of the heat for said first reheating of the vapor to the vaporat a location of the combustion gas stream downstream of the location ofthe second reheating of the vapor, controlling the temperature of thefirst reheat by regulating the recirculating gases into the furnacedirectly and predominantly in response to this temperature andsimultaneously controlling the temperature of the second reheat byadjusting the zone of combustion in the furnace with relation to thefurnace outlet directly and predominantly in response to this last namedtemperature.

5. The method of operation of claim 4- wherein the cycle of operation issuch that both of the reheats have a characteristic of increasingpercentage of the total heat absorption with decreasing load with theincrease of the high pressure reheat being at a greater rate than thelow pressure reheat and wherein the zone of combustion is moved in adirection to increase the temperature of the gases leaving the furnaceand the recirculation of combustion gases is increased as the load isdecreased to control the two reheat temperatures.

6. The method of claim 4 wherein the cycle of operation is such that ofthe total heat absorption of the unit the percentage of the first reheatincreases as the load is decreased from maximum and that of the secondreheat decreases as the load is decreased from maximum and wherein thezone of combustion is moved so as to decrease the temperature of thecombustion gases passing from the furnace as the load is decreased frommaximum and the recirculation of the combustion gases to the furnace isincreased as load is decreased from maximum to control the two reheattemperatures.

7. In a vapor generator operating on the double reheat cycle with thecharacteristic such that the high and low pressure reheat temperaturestend to decrease with decrease in load from the maximum and tend to varyrelative to each other with variation in load, said generator includinga fluid cooled furnace into which fuel is introduced and burned tocreate a combustion gas stream, the high pressure and low pressurereheaters being disposed in the gas stream, means for recirculating tothe furnace combustion gases which have traversed the reheaters and in amanner to increase the heat content of said stream of gases with thisincrease in heat content having a progressively greater effect on heatexchange surface progressively further downstream in a gas flow sense,and means, independent of the firing rate and operable simultaneouslywith the gas recirculation, eifective to adjustably vary the temperatureof the combustion gas stream issuing from the furnace, said reheatersbeing disposed in said stream so that of the last two named means one ofthe reheaters is affected predominately by the temperature varying meanswhile the other is affected primarily by the gas recirculation means,means responsive directly and predominantly to the temperature of thevapor of said one reheater effective to regulate the means to vary thetemperature of the combustion gases issuing from the furnace and meansresponsive directly and predominantly to the temperature of the vapor ofsaid other reheater effective to regulate the recirculation ofcombustion gases, whereby these vapor temperatures are maintained attheir desired value at variation in load.

8. In a vapor generator operating on the double reheat cycle andincluding a fluid cooled furnace in Which fuel is burned creating acombustion gas stream and having the characteristic that of the totalhe-a-t absorption of the generator the percentage required by thereheaters to maintain desired temperature varies relative to each otherwith varying load, the method of operation comprising effectivelyimparting heat from said combustion gas stream to the high pressurereheat vapor at one location and to the low pressure reheat vapor atanother location spaced in a combustion gas flow sense from said onelocation, simultaneously providing a pair of control actions effectiveto independently control the two reheat temperatures relative to eachother with varying load including regulatingly introducing into thefurnace combustion gases that have traversed the reheats and in a mannerto increase the heat content of the gas stream over what it wouldotherwise be and, in addition thereto and independent of the firingrate, regulatingly adjusting the temperature of the stream of combustiongases egressing from the furnace, the relative disposition of the tworeheats in the gas stream being such that the introduction of combustiongases into the furnace has a greater effect on one of the reheats thanthe other while varying the temperature of the combustion gases has agreater effect on said other than said one reheat, sensing thetemperature of the vapor of said one reheat and directly andpredominantly in response thereto regulating the introduction ofcombustion gases into the furnace to maintain this vapor temperature atits desired value and sensing the temperature of said other reheat anddirectly and predominantly in respnose thereto regulating thetemperature of the combustion gas stream egressing from the furnace tomaintain this vapor temperature at its desired value.

References Cited in the file of this patent FOREIGN PATENTS 793,048Great Britain Apr. 9, 1958 jd,ll

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No, 3,155,076 November 3, 1964 William H. Clayton et a1.

It is hereby certified that error appears in the above numbered patentrequiring correction and that the said Letters Patent should read ascorrected below.

Column 8, lines 4 and 5, and line 6, for "connection", each occurrence,read convection same column 8, line 21, strike out "and independentlybut simultaneously tom",

Signed and sealed this 16th day of March 1965.

(SEAL) Attest: l

ERNEST W, SWIDER' EDWARD J. BRENNER Attesting Officer Commissioner ofPatents

1. IN A POWER PLANT INSTALLATION A VAPOR GENERATOR OPERATING IN THEDOUBLE REHEAT CYCLE, SAID GENERATOR INCLUDING AN ELONGATED FURNACEHAVING HEAT EXCHANGE TUBES ON ITS WALLS AND HAVING A COMBUSTION GASOUTLET, MEANS FIRING SAID FURNACE IN A MANNER SUCH THAT THE ZONE OFCOMBUSTION MAY BE REGULATINGLY MOVED TOWARD AND AWAY FROM SAID OUTLET, AHIGH PRESSURE AND A LOW PRESSURE REHEATER MEANS DISPOSED SO THATCOMBUSTION GASES GENERATED IN SAID FURNACE PASS THEREOVER, MEANS FORREGULATINGLY INTRODUCING COMBUSTION GASES WHICH HAVE TRAVERSED SAIDREHEATER MEANS INTO SAID FURNACE AT A LOCATION SUCH AS TO INCREASE THEHEATING EFFECT OF THE COMBUSTION GASES PASSING OVER THE REHEATERS OVERWHAT IT WOULD BE WITHOUT SAID RECIRCULATION, SAID HIGH PRESSURE AND LOWPRESSURE REHEATERS BEING POSITIONED IN THE GAS STREAM SO THAT THECONTROL EFFECT OBTAINED BY GAS RECIRCULATION IS SUBSTANTIALLY GREATER ONTHE HIGH PRESSURE REHEATER THAN ON THE LOW PRESSURE REHEATER WHILE THECONTROL EFFECT OBTAINED BY ADJUSTING THE ZONE OF COMBUSTION WITHRELATION TO THE FURNACE OUTLET IS SUBSTANTIALLY GREATER ON THE LOWPRESSURE REHEATER THAN ON THE HIGH PRESSURE REHEATER MEANS OPERATIVE TOVARY THE RECIRCULATION OF THE COMBUSTION GASES DIRECTLY ANDPREDOMINANTLY IN ACCORDANCE WITH THE TEMPERATURE OF THE HIGH PRESSUREREHEAT VAPOR AND MEANS OPERATIVE TO VARY THE ZONE OF COMBUSTION DIRECTLYAND PREDOMINANTLY IN ACCORDANCE WITH THE TEMPERATURE OF THE LOW PRESSUREREHEAT VAPOR.